It is desirable in injection molding to urge the molten thermoplastic resin material into contact with the mold surfaces by exerting pressure on the resin. This aids the external surface of the plastic material in assuming the precise shape dictated by the mold surface. The pressure also assists the filling of the mold space with molten resin even if the space is elongated or narrow and is normally difficult to fill.
In gas assisted injection molding, the articles are produced by injecting molten resin into the mold cavity and injecting a quantity of pressurized gas into the resin to fill out the mold cavity and form a hollow portion in the resin. The gas is preferably an inert gas such, for example, as nitrogen. Pressure is maintained on the gas in the hollow gas space within the resin until the resin has sufficiently set, whereafter the pressurized gas is released from the molded part hollow space and the molded part is removed from the mold cavity.
This gas assisted procedure is advantageous since the molded part produced utilizes somewhat less plastic material and is lighter than if the part were solid plastic. More importantly, the plastic in the gas assisted procedure will not have a tendency to shrink away from the mold walls during cooling since the internal gas pressure will keep it pressed against the walls, thereby minimizing or eliminating surface blemishes such as sink marks. Further, the gas assisted procedure eliminates the need to utilize the screw ram of the injection molding machine to pack out the mold during the molding cycle, thereby minimizing or eliminating molded in stresses in the molded part.
The gas may be introduced into the mold through the same nozzle utilized to inject the resin or may be introduced into the mold at a location remote from the resin injection nozzle by a separate gas injection mechanism. One such separate mechanism is shown for example in U.S. Pat. No. 4,740,150 in which a valve member coacts with a conical seat of a valve port opening into the mold cavity to allow introduction of gas into the mold cavity through a central bore in the valve member and to allow venting of the mold cavity by retraction of the valve member to allow the pressurized gas to escape from the mold cavity through the valve port. This arrangement suffers from the disadvantages that the seal is affected by the forced seating of the tip of the valve member in the conical seat of the valve port with resultant strain on the various component parts. Also, the various parts must be precisely machined and precisely aligned relative to each other to provide an effective seal, and the seal is readily destroyed when resin contaminants are deposited on the conical valve seat and/or on the tip of the valve member.
It has also been suggested that the separate gas injection mechanism might comprise a pin slidably mounted in a bore in the mold so that the pin may be moved between an advanced position in which the forward tip of the pin is positioned in communication with the mold cavity to allow the introduction of gas into the mold cavity through a central bore in the pin, and a retracted position in which the gas is vented through the bore. However, this arrangement suffers from the disadvantage that, with extended usage, the bore may become wholly or partially plugged with resin with the result that the pin can no longer move forwardly to its advanced position to achieve the gas injection operation.